The sex pheromone emitted by the female tick of the species Dermacentor andersoni and D. variabilitis is 2,6-dichlorophenol. This single compound elicits the complete behavioral hierarchy of orientation, mate identification and copulation. No other volatile component contributes to this behavior in the bioassay. The pheromone is emitted by the foveal glands. Supporting evidence was obtained by glc identification of 2,6-dichlorophenol in extracts of the dissected glands and as trapped material from aeration studies; this trapped material also gave strong responses in the bioassay, and when the ticks were fed on a 36Cl-labelled host, the trapped material was radioactive. When the foveal glands were blocked with laquer, no active emissions were obtained. Further support is derived from scanning electron microscopy, energy-dispersive x-ray analysis, and autoradiography. Disruption of mating activity with synthetic pheromone shows promise for tick control.